What is called diesel combustion, in which fuel is directly injected into compressed air in the combustion chamber, self-ignites, and is burned by diffusion combustion, has a higher thermal efficiency as compared to combustion by spark ignition. In recent years, in order to enjoy this advantage of diesel combustion also in gasoline engines, technology for causing gasoline to self-ignite and burn by diffusion combustion has been developed.
For example, in the technology disclosed in PTL 1, a first fuel injection is performed by a fuel injection valve in a cylinder during the first half of the compression stroke to form substantially homogeneous air-fuel mixture in the entirety of the combustion chamber. Then, the air-fuel mixture formed by the first fuel injection is spark-ignited. Thereafter, a second fuel injection is performed, and the injected fuel is burned. Moreover, the remaining fuel self-ignites with a rise in the temperature and pressure in the combustion chamber resulting from the burning.
PTL 2 discloses a technology enabling diesel combustion using as fuel natural gas or the like having a relatively high self-ignition temperature. According to PTL 2 disclosing this technology, fuel injection is performed in a predetermined spark-ignition region in the combustion chamber in an early or middle stage of the compression stroke to form air-fuel mixture that can be spark-ignited. Then, the air-fuel mixture formed in the spark-ignition region is ignited at a time immediately before the top dead center of the compression stroke to bring about combustion by spark ignition. Thus, a high-temperature, high-pressure condition enabling self-ignition of natural gas is established in the combustion chamber. Thereafter, fuel is injected directly into the combustion chamber in a high-temperature, high-pressure condition, so that the injected fuel is burned by diesel combustion.
In the case where an internal combustion engine that performs diesel combustion is equipped with a supercharger for supercharging the intake air, when the internal combustion engine is in an operation state in which the engine load is increased or decreased, there is a response delay in changing the boost pressure (supercharged pressure) in response to a change in the fuel injection quantity. If the air-fuel ratio of air-fuel mixture deviates from a desired range due to such a response delay in changing the boost pressure, there may arise a possibility of increase in the amount of smoke generated and a possibility of instable diesel combustion. PTL 3 discloses a technology for solving this problem arising in the operation state in which the engine load is increased or decreased in a supercharged diesel engine. More specifically, in the technology disclosed in PTL 3, gradual control by which the fuel injection quantity is changed gradually during transient operation.